Turbochargers are generally known and used for increasing power in both internal combustion and diesel engines. Exhaust gas is used to provide energy for a turbine, and the turbine powers a compressor. The compressor receives air from atmosphere, compresses it, and forces the air into the intake manifold of the engine. The turbine includes a turbine wheel located in a turbine housing, and the compressor includes a compressor wheel located in a compressor housing; both the compressor wheel and the turbine wheel are mounted on a common shaft forming a rotor assembly. The shaft features a bearing system positioned in an intermediate housing between the compressor and the turbine which allows the shaft to rotate at very high speeds.
Typical bearing systems for these shafts employ the use of hydrodynamic oil films, these bearing systems have clearances between the intermediate housing and the bearing, and the bearing and the shaft. The clearance between the bearing and the intermediate housing is typically 3.5 to 4 times larger than the clearance between the bearing and the shaft. The reason for this proportion in clearances is due to the hydrodynamic forces that act on the oil films. The clearances allow the shaft to tilt inside the bearing housing during operation; because of the tilting of the rotor assembly, the clearances between the turbine wheel and the turbine housing and the compressor wheel and the compressor housing must be large enough to prevent the turbine wheel from coming into contact with the turbine housing, and the compressor wheel from coming into contact with the compressor housing. One problem that exists with this type of configuration is that larger tolerances between the compressor wheel and the compressor housing, and between the turbine wheel and the turbine housing, reduce the overall efficiency of the turbocharger.
Many attempts have been made to introduce the use of ball bearings into turbochargers because of the improved response time, improved friction, and improved stiffening of the rotor assembly, thereby allowing tighter tolerances between the compressor wheel and the compressor housing, as well as between the turbine wheel and the turbine housing, improving the overall efficiency of the turbocharger. A major significant drawback to the use of ball bearings in a turbocharger assembly is that because of the higher levels of heat and, more importantly, the higher shaft speeds that overstress the rolling elements in the bearing, ball bearings often fail when used in a turbocharger assembly.
Accordingly, there exists a need for a turbocharger which can incorporate the advantages of a ball bearing such as the improved response time, improved friction, and improved stiffening of the rotor assembly while at the same time avoiding bearing failure.